ci: port scripts/ci/fat_driver.py to Rust (pmat complexity hook refuses any edit to it) #5741

scripts/ci/fat_driver.py has 14 complexity errors on main (pmat analyze complexity --max-cyclomatic 30 --max-cognitive 25, e.g. postfix cog 182, _run cog 74, loose_eq cog 56). The pre-commit hook judges whole files, so no edit to it can be committed. PMAT-5740 (#5740) worked around this with a Rust subcommand in aprender-ci-tools. Long-term: port the driver to Rust (C301: no new Python), then retire the .py. Refs #5510.